1. Field of the Invention
This invention relates to a method of picking out a synchronizing light beam in a light scanning system, and more particularly to a method of picking out a synchronizing light beam separated from a signal light beam with which the former is synchronized in a light scanning system.
2. Description of the Prior Art
Light scanning systems are well known and used in various optical systems such as optical beam recording, oscillograph tracing, television line generation, detection of surface imperfections of webs and so forth. In such light scanning systems, galvanometer type vibrating mirrors or rotating mirrors of polygonal structure are often used as a light scanner. The light scanner is required to be driven in accurate synchronization with the recording medium or the web detected so that the scanning of the light beam thereon may be accurately synchronized with the movement thereof in the direction perpendicular to the direction of scanning. When the brightness of the light spot of the scanning light beam formed on the recording medium or the web is to be indicated in an oscilloscope or a recorder or put in an electronic computer as an input signal, it is necessary to supply a synchronizing signal accurately synchronized with the scanning light beam to these output devices. Particularly when an image is displayed or recorded, it is highly essential that the image signal be accurately synchronized with the position of the light spot of the scanning light beam which is detected or indicated by the synchronizing light beam.
In order to obtain a synchronizing signal which indicates the position of the light spot of the scanning light beam formed by the rotating mirror, a variety of methods employing magnetic, electric or photoelectric means have been employed to detect the state of rotation of the rotating mirror. As one of the methods of detecting the state of rotation of the rotating mirror employing a photoelectric means, it has been known in the art to make a light beam impinge upon a face of the rotating mirror used to deflect the signal light beam and cause the light beam deflected by the rotating mirror to be detected as a synchronizing light beam. Methods of photoelectrically detecting the state of rotation of the rotating mirror are divided into two groups in one of which the signal light beam used for displaying or recording images and the synchronizing light beam are made by a single light source and in the other of which the signal light beam and the synchronizing light beam are made by different light sources.
In the conventional method of photoelectrically detecting the state of rotation of the rotating mirror by use of a single light source, the light beam emitted by a single light source is divided into a signal beam and a synchronizing beam by a beam splitter before the light beam is modulated and then the signal beam and the synchronizing beam are deflected by different facets of the rotating mirror. In this method, there are defects in that the correction of errors in the angle of facets in the direction of scanning is difficult and dust proof casing for covering and protecting the rotating mirror must be provided with a large opening for the synchronizing light beam. Further, it has also been known in the art to make the signal beam and the synchronizing beam divided from a light beam from a single light source impinge upon a single facet of the rotating mirror and to detect the synchronizing beam by use of a photoelectric detector located at a position where the detector does not obstruct the scanning signal beam for forming an image. In this method, there are defects that particular care must be taken to prevent crosstalk between the signal beam and the synchronizing beam and the photoelectric detector must be located in the very limited space where the light scanner and other optical systems are arranged. Further, both these methods are disadvantageous in that the intensity of the signal light beam is reduced by the beam splitter.
In the other conventional method of photoelectrically detecting the state of rotation of the rotating mirror by use of different light sources, the crosstalk can be comparatively easily prevented by making the wavelength of the signal beam different from that of the synchronizing beam. However, in this method, there is a defect in that the arrangement of various optical and electrical elements is considerably difficult since the light source of the synchronizing light beam, a converging optical system for the synchronizing beam and a photoelectric detector must be located in the same small space occupied by a light source of the signal light beam, a converging optical system for the signal beam and the rotating mirror. In addition, even in this method there is a possibility of crosstalk when the optical path of the signal beam and that of the synchronizing beam are close to each other. Therefore, this method requires use of a condenser lens or a filter in order to pick out the synchronizing beam with sufficiently high signal-to-noise ratio.
In the light scanning systems employing a rotating mirror, a spot motion stabilizing means is usually provided to stabilize the spot motion perpendicular to the scan direction caused by the error in manufacture of the faces of the rotating mirror. Since a highly accurate mirror having a sufficiently small face-to-axis of rotation tolerance is both difficult to manufacture and expensive, the scanning system employing a rotating mirror is usually provided with a spot motion stabilizing means comprising a combination of cylindrical lenses as disclosed e.g. in U.S. Pat. No. 3,750,189. The spot motion stabilizing means uses an optical system of complicated structure which occupies a large space around the rotating mirror. Therefore, in the light scanning systems wherein a spot motion stabilizing means is employed together with a rotating mirror, it is particularly difficult to arrange the various optical elements around the rotating mirror.